A goal of many modem long haul optical transport systems is to provide for the efficient transmission of large volumes of voice traffic and data traffic over trans-continental distances at low costs. Various methods of achieving these goals include time division multiplexing (TDM) and wavelength division multiplexing (WDM). In time division multiplexed systems, data streams comprised of short pulses of light are interleaved in the time domain to achieve high spectral efficiency, high data rate transport. In wavelength division multiplexed systems, data streams comprised of short pulses of light of different carrier frequencies, or equivalently wavelength, are co-propagate in the same fiber to achieve high spectral efficiency, high data rate transport.
The transmission medium of these systems is typically optical fiber. In addition there is a transmitter and a receiver. The transmitter typically includes a semiconductor diode laser, and supporting electronics. The laser may be directly modulated with a data train with an advantage of low cost, and a disadvantage of low reach and capacity performance. After binary modulation, a high bit may be transmitted as an optical signal level with more power than the optical signal level in a low bit. Often, the optical signal level in a low bit is engineered to be equal to, or approximately equal to zero. In addition to binary modulation, the data can be transmitted with multiple levels, although in current optical transport systems, a two level binary modulation scheme is predominantly employed.
Typical long haul optical transport dense wavelength division multiplexed (DWDM) systems transmit 40 to 80 10 Gbps (gigabit per second) channels across distances of 1000 to 6000 km in a single 30 nm spectral band. A duplex optical transport system is one in which traffic is both transmitted and received between parties at opposite end of the link. In current DWDM long haul transport systems transmitters different channels operating at distinct carrier frequencies are multiplexed using a multiplexer. Such multiplexers may be implemented using array waveguide (AWG) technology or thin film technology, or a variety of other technologies. After multiplexing, the optical signals are coupled into the transport fiber for transmission to the receiving end of the link.
At the receiving end of the link, the optical channels are de-multiplexed using a de-multiplexer. Such de-multiplexers may be implemented using array waveguide (AWG) technology or thin film technology, or a variety of other technologies. Each channel is then optically coupled to separate optical receivers. The optical receiver is typically comprised of a semiconductor photodetector and accompanying electronics.
The total link distance may in today's optical transport systems be two different cities separated by continental distances, from 1000 km to 6000 km, for example. To successfully bridge these distances with sufficient optical signal power relative to noise, the total fiber distance is separated into fiber spans, and the optical signal is periodically amplified using an in-line optical amplifier after each fiber span. Typical fiber span distances between optical amplifiers are 50-100 km. Thus, for example, 30 100 km spans would be used to transmit optical signals between points 3000 km apart. Examples of in-line optical amplifers include erbium doped fiber amplifers (EDFAs) and semiconductor optical amplifiers (SOAs).
The architecture of current optical transport systems comprise a high degree of specialization. For example, the receiver line card is often separated from the transmitter line card so that the two cards are required at each terminal to achieve one channel of duplex operation. This configuration is inefficient in its use of space, power and logistical operation, and there is a need for an integrated line card with high density.
A further limitation in the current art is the inflexibility of current transceiver cards. For example, in the current art, a transceiver card that supports the SONET standard, cannot support the Ethernet standard. Further, in the current art, a transceiver card that supports 4 OC48 SONET signals cannot support an OC192 SONET signals despite the fact that both of these signals have the same aggregate data rate of approximately 10 Gbps. There is, consequently, a need for a transceiver line card that is flexible to operate at different standards.
A further limitation in the current art is the inflexibility of current transceiver cards to support different Forward Error Correction (FEC) standards. For example, in the current art, a transceiver card that supports a G.709 FEC with 7% overhead cannot support an extended FEC with 25% overhead. There is, consequently, a need for a transceiver line card that is flexible to support different FEC standards.
Another limitation in the current art is the inflexibility of current transceiver cards to support different optical performances and capabilities. For example, a transceiver card that could be upgraded from the field to incorporate a tunable laser and be re-used in another location is not currently possible in the art. Furthermore, the mixing and matching of different optical reach performances (and associated costs) in the same systems is desirable by the industry but not available in the art of DWDM long haul transport systems. From a competitive perspective, the technology of the line optics portion of transceiver cards is often a critical driver to an optical transport system's competitive advantage through the incorporation of either higher performance components or lower cost components. There is consequently a need for a transceiver line card that is flexible to support tunable lasers, enhanced system performance, or cost reduction means through easy incorporation of state of the art line optics components.
There are other limitations in the current art related to manufacturability and reliability of transceivers in optical systems. Transceivers of the prior art comprise a single large complex card with thousands of components. They must be manufactured and assembled in many stages before functional testing can be accomplished. The recognition of component failure during the late functional testing requires a complex and expensive rework process or scrapping the entire assembly. Since reliability of an entity decreases as the number of components increase, it is desirable to reduce the number of components per testable entity in the manufacturing process and in the final product. It is also desirable to make groups of these components field replaceable. There is consequently a need for a transceiver line card architecture that is functionally decomposed into a few integrated parts for manufacturability, testability, reliability, and for inventory reduction through the mix and match of the tested parts.
In the prior art, a single microcontroller and power supply is required per optical channel. The invention architecture maximizes the number of optical channels per line card to reduce cost, power, and space; and to increase channel density. For example, only a single controller and power supply are required for up to four channels.